A cartilage tissue is a supporting tissue consisting cartilage cells and an extracellular matrix surrounding them, which is present for alleviating friction generated between bones and absorbing impact in a joint such as a knee joint. When a cartilage matrix forming the cartilage tissue is degenerated due to a variety of causes, water content is so reduced that its function cannot be maintained, leading to onset of osteoarthritis (OA), which is an arthritic disorder associated with chronic arthritis. It is a disease which degenerates joint components, leading to destruction of a cartilage as well as proliferative change of a bone and a cartilage. The total number of patients with osteoarthritis in Japan is estimated to be about eight millions, and the patients are expected to further increase as the population ages.
Once osteoarthritis develops damage and destruction of a cartilage and a bone, they cannot be restored to their original state later. However, early detection and proper treatment can delay the progression of symptoms. Appearance and development of symptoms of osteoarthritis differ greatly in individuals. For selecting proper treatment, it is, therefore, crucially important that each patient is early and thoroughly examined for the condition of an articular cartilage to find abnormalities. Likewise, aside from clinical practice, it is also crucially important for developing a highly effective therapeutic agent for degeneration of an articular tissue, at least degeneration of an articular cartilage of an experimental animal can be evaluated qualitatively and quantitatively, and furthermore, wherever possible, in the living state (in vivo) over time.
Currently, articular examination in a human patient is conducted generally by plain X-ray photography, arthrocentesis, arthroscopy or the like. Plain X-ray examination is inexpensive and practicable for any medical institution, but an articular cartilage cannot be imaged by X-ray examination because main components of an articular cartilage are aggrecan as a proteoglycan containing chondroitin sulfate and keratan sulfate side chains and collagen. It can be applied to an experimental animal. Therefore, X-ray photography can determine the degree of articular destruction by checking bone change around the joint such as narrowing of a cleft between articulations (cleft between two bone ends which mutually face in a joint), but can only conduct indirect evaluation for change in the cartilage itself. In other words, X-ray photography cannot directly detect the degree of actual destruction or degeneration of the cartilage, and therefore, cannot quantify the degree or find destruction of the articular cartilage in the mildly symptomatic state. Arthrocentesis as an alternative method can determine the state of an articular cartilage by the use of physiological or biochemical changes as indicators, but is ineffective for determining physical conditions such as a thickness and deformation of an articular cartilage. Furthermore, an articular cartilage can be directly imaged for diagnosis by arthroscopic methods. These include a method for determining physical properties of a cartilage by irradiating a cartilage tissue with a laser from the tip of an arthroscope and then detecting ultrasound generated from the tissue (see Patent Reference No. 1) and a method for objectively evaluating the degree of degeneration of a cartilage from an early stage by determining variation over time in an absorbance associated with compressional deformation of the cartilage by the use of a near-infrared aquameter (see Patent Reference No. 2). Any of these methods, however, highly invasive and thus has a problem of imposing a heavy physical burden and infection risk on a patient. These methods can be, therefore, applied to limited cases for a human patient, and even for an experimental animal, these methods are less applicable because such invasion makes it difficult to perform serial examination required for drug evaluation for an arthritic disorder.
On the other hand, MRI has been increasingly employed cartilage imaging in a human patient and is expected to be as an examining means which allows for qualitative evaluation of a cartilage itself. An MRI machine is, however, so expensive that a limited number of medical institutions can introduce an MRI machine. Furthermore, its unsatisfactory resolution also makes it difficult to use the device for the above purpose.
In these circumstances, attempts have been made for developing a method for early diagnosis of the condition of a cartilage and an accurate disease marker therefor (Patent Reference Nos. 3 and 4).
Meanwhile, a fluorometric imaging device for conducting optical projection tomography (OPT) of a body tissue ex vivo using a fluorescent molecule has been developed as technique for selectively constructing a three-dimensional image of a tissue inside the body. According to this technique, a fluorescently-stained living tissue is irradiated with pulse laser as excitation light, and for each pulse irradiation, photons generated from the irradiated site in the living tissue are amplified by a photomultiplier and then detected. The detected signal is processed by a time-correlated single-photon counting method, and the resulting data can be imaged to provide any sectional image of the target tissue or an image of the whole tissue (three-dimensional image, sectional image). Furthermore, an in vivo fluorometric imaging system has been recently developed and commercially available, which can detect the location of a fluorescently-labeled substance in the body of an small animal such as a living rat or mouse from the outside for imaging (for example, GE HEALTHCARE, “eXplore Optix”). According to the system, a fluorescent label which specifically accumulates in a target tissue is administered and its three-dimensional distribution can be determined over time to be imaged. In vivo fluorometric imaging, which is noninvasive and thus safe and has high sensitivity has been increasingly employed to image a marked particular tissue or its component in a living experimental animal over time for evaluating the kinetics of a protein or change in the status of a lesion, and is expected to be similarly utilized for a human tissue in the future.
There has been recently reported a cartilage marker utilizing the property that a polyarginine peptide (a) or polylysine peptide (b) having the structure shown below is specifically bonded to a cartilage tissue (see Patent Reference No. 5). These polypeptides have a structure that an α-amino group and a carboxyl group in arginines or lysines are bonded via a peptide bond, as shown in the following formulas. A compound described in Patent Reference No. 5 is a cartilage tissue marker in which a fluorescent group or an X-ray absorbing group is bonded to an N-terminus or C-terminus.

For the compound described in Patent Reference No. 5, in the case of introducing a fluorescent substance, a high-sensitive fluorescent group can be introduced to visualize a desired cartilage tissue. Meanwhile, when an X-ray absorbing substance is used, it must contain many X-ray absorbing atomic group such as iodine atoms. However, a polyarginine peptide or polylysine peptide has a large molecular weight, so that the number of iodine atoms to be introduced in one oligomer molecule must be large. It can affect solubility of the polyarginine peptide or the polylysine peptide or permeability of the compound in a cartilage tissue. Thus, there is room for improvement.
Patent Reference No. 6 has described an E-polylysine in which an amino group is protected by an urethane bond. Herein, an ε-polylysine has a structure that an ε-amino group and a carboxyl group of lysines are linked via a peptide bond. Patent Reference No. 6 has described that such an ε-polylysine is used for bathroom furnishings, cosmetics, feed additives, medical drugs, pesticides, food additives, electronic materials or the like. There are, however, no descriptions about the use of such an ε-polylysine as a marker for a living tissue or about introduction of a fluorescent group or a group for radiographic visualization to an ε-polylysine.